Human immunodeficiency virus (HIV) isolated from patients treated with zidovudine (AZT) may demonstrate markedly reduced in vitro susceptibility to AZT (Larder et al., 1989, Science 243:1731–1734; Rooke et al., 1989, AIDS 3:411–415; Land et al., 1990, J. Infect. Dis. 161:326–329; Boucher et al., 1990, Lancet 336:585–590; Japour et al., 1991, Proc. Natl Acad. Sci. 88:3092–96; Tudor-Williams et al., 1992, Lancet 339:15–19). This reduced susceptibility has been related to the duration of therapy with AZT and the severity of HIV disease at the time AZT therapy is begun (Richman et al., 1990, J. AIDS 3:743–756). Nucleotide sequence analysis of AZT-resistant HIV strains has revealed a number of mutations in the reverse transcriptase (RT) gene associated with decreased AZT susceptibility (Larder et al., 1989, Science 246:1155–1158; Larder et al., 1991, AIDS 5:137–144; Kellam et al., 1992, Proc. Natl. Acad. Sci. USA 89:1934–1938; St. Clair et al., 1991, Science 253:1557–1559; Richman et al., 1991, J. Infect. Dis. 164:1075–1081). Molecular cloning experiments have confirmed that these mutations in the RT gene confer AZT resistance (Larder et al., 1989, Science 246:1155–1158; Larder et al., 1991, AIDS 5:137–144; Kellam et al., 1992, Proc. Natl. Acad. Sci. USA 89:1934–1938; St. Clair et al., 1991, Science 253:1557–1559). Of these mutations the one at codon 215 resulting in a single amino acid substitution (Thr→Tyr or Phe) has been shown to be the most common mutation and to have the greatest impact on in vitro susceptibility to AZT (Larder et al., 1991, AIDS 5:137–144; Richman et al., 1991, J. Infect. Dis. 164:1075–1081; Boucher et al., 1992, J. Inf. Dis. 165:105–110).
Several studies have addressed the relationship between in vitro AZT resistance, mutations in the RT gene and clinical disease. Richman and coworkers studied 32 patients with different stages of HIV disease and demonstrated that the development of in vitro AZT resistance was related to the duration of therapy with AZT and to the severity of disease at the time AZT was begun (Richman et al., 1990, J.AIDS 3:743–746). Boucher and coworkers studied HIV P24-antigenemic patients treated with AZT for 2 years. They observed that at 6 months, seven patients with a mutation at codon 215 had a weak, non-statistically significant trend toward lower CD4 counts compared to nine patients who were wild type at codon 215 (Boucher et al., 1990, Lancet 336:585–590). After 2 years nearly all patents had the mutation. Tudor-Williams and coworkers studied HIV isolates from 19 symptomatic children treated with AZT for 9–39 months and showed that in vitro AZT resistance was associated with poor clinical outcome (Tudor-Williams et al., 1992, Lancet 339:15–19). However, adult studies have not shown a precise correlation between the development of in vitro resistance and progression of HIV disease.